Mechanical stresses in the form of compression or tensile stresses are produced with uneven heating over the surfaces of flat or curved glass or glass ceramic plates. The level of these stresses depends on the temperature distribution on the plate and on physical material properties, such as, e.g., thermal expansion coefficients, the modulus of elasticity, heat conductivity, etc. If the tensile or compression stresses exceed the maximum permissible limits that are set by the strength of the plate, the plate will break.
Plates of this kind that are made of glass or glass ceramic with unevenly heated areas are used as, e.g., stove cooking surfaces (electric, gas, induction and solid-fuel stoves), grilling surfaces, light covers, heating element covers, etc.
In principle, the glass or glass ceramic plates can have a positive or a negative thermal expansion coefficient. If the thermal expansion coefficient is negative in the temperature range in question, which should be the exception rather than the rule, then tensile stresses will arise in the flat areas of the plate, where the latter is heated. Conversely, in the case of positive thermal expansion coefficients, which occur more frequently, the tensile stress arises in the colder edge areas that adjoin the heated areas, which can make these edge areas vulnerable to breaking. Moreover, because of the mechanical machining of the edge, the edge area of a plate in any case has lesser strength. The risk of breakage being caused by the tensile stresses that arise, especially in the less strong edge areas, means that certain materials are ruled out for some of the above-indicated possible applications or can be used only for a relatively narrow temperature range.
To overcome these drawbacks, in the previous prior art the glass or glass ceramic was subjected to hardening (prestressing), whereby compression stress was produced in the surface layer of the glass product and tensile stress was produced in the interior. In this case, the compression stresses on the surfaces of the glasses increase their strength since, in the presence of tensile stress, these compression stresses on the surface must first be overcome before the formation of tensile pressure peaks ultimately leads to breakage.
For tempering (prestressing) of glass items, basically two processes are available: thermal tempering and chemical tempering.
With thermal tempering, glasses are heated to just below their softening point and are then quickly cooled. Because the interior of the glass cools more slowly than the surface, the surface is placed under compression stress, while the interior is exposed to tensile stress. Since the effects that occur are greater, the greater the thermal expansion of the glass, this process is limited to materials with fairly large expansion coefficients. Another drawback consists in the fact that the increases in hardness are quickly destroyed when the glass is brought to temperatures of its transformation range, so that the process cannot be used on objects that are exposed during use to temperatures that exceed the relaxation temperature of the thermally tempered (prestressed) glass. Moreover, thermal tempering is less efficient in the case of thin-walled objects.
Chemical tempering is based on a compression prestress being produced in the glass surface by altering its chemical composition relative to the glass interior, whereby surface layers with lower thermal expansion coefficients or larger volumes than the interior of the glass are produced. The chemical tempering method consists of an ion exchange. It has the drawback, however, that it is a comparatively time-consuming and expensive process since only layers that are too thin and are under compression stress are produced within economically justifiable periods. Moreover, the method is limited since it can be used only on glasses of specific chemical composition. Another drawback consists in that the compression stress layer has a chemically tempered glass of typically only a thickness of 100 to 200 .mu.m, such that the process is limited to applications in which large surface damage would penetrate the compression stress layer, which would obviate the protective action.